DESCRIPTION (adapted from the application) A primary problem in clinical transplantation is the inability to induce long-lasting allograft tolerance. Current immunosuppressive medications offer excellent one-year graft survival but long-term survival is limited by ongoing immunologic and hemodynamic damage to the graft. Therefore a major goal of transplant physicians has been to design therapeutic strategies that induce tolerance to alloantigens and confer long-lasting graft and patient survival. Many investigators have shown that disruption of critical molecules involved in T cell-antigen presenting cell interactions can confer tolerance. Distinct alterations in TCR-mediated biochemical signals have been observed in vitro using T cell clones that have been rendered unresponsive by altered peptide ligand or costimulatory blockade. We have developed, in the first two years of a K08 award, an in vivo model of allotolerance in which T cell unresponsiveness is induced by antigen exposure. We have begun to define the alterations in biochemical signals that result in T cell unresponsiveness in the murine TCR transgenic model. This work has led to oral presentations at several national and international meetings and publication of the initial findings. The model system may provide a better understanding of how to confer alloantigen tolerance and may provide new, targeted approaches to immunosuppression. Through the R03 grant, the initial findings in aim 1 of the K08 will be expanded to focus on the role of the MHC-peptide complex in induction of unresponsiveness and to further clarify the physiological significance of the model. The specific focus of aim 2 is to now analyze the positive and negative regulatory events operative in the induced state of unresponsiveness. The hypothesis is that T cell unresponsiveness induced by antigen exposure is linked to specific defects in TCR-mediated intracellular signaling pathways. The proposed experiments will allow a mechanistic evaluation of intracellular targets and clarify the relevant positive and negative regulatory components. The R03 proposal is linked to the Mentored Clinical Scientist Development Award (K08-DK02472). The PI is in her third year of the K08 award. The preliminary data obtained through the R03 proposal will be used to facilitate publication of work currently in progress and support the PI's application for an independent investigator award (R01).